Growth of Japanese scallop Mizuhopecten yessoensis farmed by the ear-hanging method between different water layers and estimation of the productivity for farmed scallops in Ogatsu Bay (northeastern Japan) following the 2011 Great East Japan Earthquake

This study evaluated the productivity of farmed bivalves in Ogatsu Bay after the tsunami disaster in 2011. We focused on the Japanese scallop farmed by the ear-hanging method to analyze the growth status between different water layers and its productivity. We also monitored marine environmental changes (seawater temperature, chlorophyll a concentration) at neighboring farming sites in order to assess the feeding environment. As a result, the productivity decreased in 2014. This drop was mainly due to the increase of farming facilities and the stagnation of production amount in 2014. Since chlorophyll a concentration (i.e., phytoplankton) did not decrease from 2013 to 2014, the drop in 2014 was suspected to be due to overproduction of the farmed scallop. Specifically, significant differences were seen in shell size, soft body weight, and adductor muscle weight between the upper and lower layers in 2013 and notably in 2014. Therefore, it seems that the increase of farming facilities caused feeding competition in the lower layer, resulting in the decline of growth rate. These results indicate that the present number of scallop farming facilities may be already above the sustainable carrying capacity in Ogatsu Bay for the best growth performance.